Challenges exist in obtaining solar cell back surface fields (bsfs) on a low-to-moderate thermal budget (for example, <400-800 C) in p-type silicon (Si). Aluminum-(Al-)based bsfs (fabricated by >800 C alloying of an Al-paste or metallic Al layer) can have the desired several-micron thickness, but can be difficult to form at temperatures below 800 C due to the low solid solubility of Al in Si. While boron (B) dopants have much higher solid solubilities in Si, the long periods at high temperatures (for example, 900-1050 C) required for sufficient B diffusion can degrade the bulk lifetime of the Si substrate and/or lead to dopant clustering in ways that can produce misfit dislocations.
Some back surface field functionality can be provided in heterojunction with intrinsic thin layer (HIT) cells with the use of intrinsic amorphous silicon (i-aSiH)/doped-aSiH stacks on Si substrates (for example, p-Si(substrate)/i-aSiH/p-aSiH and n-Si(substrate)/i-aSiH/n-aSiH)), but these cells can be difficult to fabricate due to the narrow process window for providing i-aSiH/doped-aSiH stacks with aSiH layers thick enough to provide good passivation yet thin enough to provide sufficient tunneling current to the back surface metallurgy.
Accordingly, a need exists for a low-temperature, easy-to-integrate technique for forming B-doped back surface fields in p-type Si.